This invention relates generally to rotating machinery, and more particularly to steam turbines, and methods and system for calculating steam path clearances in steam turbines.
A steam turbine includes a steam path which typically includes, in serial-flow relationship, a steam inlet, a turbine, and a steam outlet. At least some known turbine rotor assemblies include a plurality of rows of blades coupled to a rotor wheel. The blades are arranged in axially-spaced stages that extend circumferentially around the rotor wheel. More specifically, each stage includes a set of stationary blades or nozzles, and a set of cooperating rotating blades, known as buckets. The tips of the rotating blades are surrounded by a turbine casing such that a radial gap is defined between the rotor blade tips and the casing.
An operating efficiency of the turbine is at least partially dependent upon the radial clearance or gap between rotor blade tips and the casing. For example, if the clearance between the rotor blade tips and the surrounding casing is too large, flow may leak through the gap between the rotor blade tips and the surrounding casing, decreasing the turbine's efficiency. However, if the clearance is too small, the rotor blade tips may strike the surrounding casing during certain turbine operating conditions. To facilitate optimizing the turbine efficiency, the clearance is adjusted to enhance steady-state performance while maintaining an adequate clearance margin as the turbine accelerates and decelerates through the rotor train vibration criticals. A cold clearance, which is initially tight, can result in excessive regenerative rubs. Over time, continued rubs may cause loss of material and/or a blunt or mushroomed seal tooth which may change the flow characteristics and adversely affect the performance of the turbine. A balanced design may provide tight average operating clearances as well as facilitate avoiding rubs during transients and operating at off-design conditions.
Turbine radial clearances may change during periods of acceleration or deceleration due to changing centrifugal force induced to the blade tips, and/or due to relative thermal growth between the rotating rotor and stationary casing. During periods of differential centrifugal and thermal growth, clearance changes may result in rubbing of the moving blade tips against the stationary casing. Such an increase in blade tip clearance results in efficiency loss. Since components of steam turbines are made of different materials with different thicknesses, such components exhibit different rates of thermal growth from a cold startup condition to steady state operating condition and during transient operating conditions. Additionally, turbine components are subject to vibratory excitement during transient operation that also affects steam path clearances. Such differences make calculating steam path clearances difficult and time consuming.